Big Battery Pack

I'm putting together a battery pack involving 24 parallel protected 18650 cells.


In my mind that's a decent size battery and I'm trying to make sure I'm not missing any precautions.

My plan is to break the battery into 2 or three smaller packs so that I can charge them up together with a multi-cell balance charger at around 4-5A rate. This will involve a funny sort of plug but that's not a problem.


I'm wondering what will happen if one of the cells dies. Will the protection circuit make it safe but just lose some capacity in the overall pack? Or will it be a danger?

Do I need to add in a small indicator LED + switch so that I can regularly check each cell is working and remove any that fail?


What am I missing to make this big 3.7V battery pack work well and safely?

generally, one will build with Unprotected cells (finish with a protecting circuit) ....... i did also .....

what type of multi cell charger are u using?

at 24 battery pack, if 1C = 2amps, u will have about 48Amps ... kindly ground yrself when handling the pack poof amps kill not voltage ...

the only way i can think of 'checking' individual cell is checking manually one by one ....... (disconnect from the 24 pack, amps kill)

what brand/type of cells are u using??

the biggest parallel pack i see is 4 to 6 cells ...... (cylinder cells)

normally, big packs are built with Li polymer or A123s or LiMnNi ......

and charge it out side on concrete in some one elses back yard. wht not try battryspace.com they sell pack and large lithium polymer cell.

petersmith6 said:

and charge it out side on concrete in some one elses back yard. wht not try battryspace.com they sell pack and large lithium polymer cell.

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I've looked at those packs.

The equivalent of what I want is http://www.batteryspace.com/polymerli-ionbattery37v50ah185wh7arate.aspx which costs $300. I bought my cells for about $70.

Theirs also has a 7A rate and I need at least 10A.

That many battery packs together is a possible time bomb. At a minimum I would charge each cell individually and then do a discharge capacity test. Any cells that are more than 10% out of spec, or average for the group, I would not use. Also don't use individual cell protection. If they are all parallel it's pretty much a self balancing pack so you only need one protection circuit for the whole pack. I don't know how much current output you need so the protection circuit may need to be custom. Also unless you have a cell welder I really don't recommend soldering the packs. One mistake and you can destroy the polymer separator in the cell and next time you charge it

As for safety the pack won't shock you since it's all in parallel so it's only 4.2 volts max. Amps do kill but they need enough voltage to pass through the body. Under extremely rare ideal circumstances you need at least 30 volts to pass enough current to kill. The pack does not have 48 amps since amps is a measure of current only, it has 48 amp-hours of capacity.

roadie said:

at 24 battery pack, if 1C = 2amps, u will have about 48Amps ... kindly ground yrself when handling the pack poof amps kill not voltage ...

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A car battery puts out 12 volts and can put hundreds of amps - have you ever heard of someone being electrocuted by one ???

How would grounding reduce the risk ????

Please check on you facts before giving such advice.

I've just built up an 8 x 18650 pack using protected cells.

Protected cells already have solder tabs on both terminals, but the insulation between the strap from the Positive Cap and the case is VERY thin, so you have to be very careful.

I used protected cells, because if one cell dies, the protection circuit will isolate it.

MikeAusC said:

A car battery puts out 12 volts and can put hundreds of amps - have you ever heard of someone being electrocuted by one ???

How would grounding reduce the risk ????

Please check on you facts before giving such advice.

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i would be delighted to see u touching a circuit such as a 12v sla or this project when the circuit is alive ......

btw, "A car battery puts out 12 volts and can put hundreds of amps - have you ever heard of someone being electrocuted by one ???" =>>>>>>> no insane man will be stupid enough to handle the sla when its running ..... if u dun believe, try it yrself.....

a good grounding of oneself is a basic personnel good protection when handing high voltage or amps .......

yes, while a battery is stable enough to handle (usually) while not connect to any circuit as it will need something to "draw" volts or amps ....... But thats an different case when the circuit is alive ,,,,,,,

MikeAusC said:

I've just built up an 8 x 18650 pack using protected cells.

Protected cells already have solder tabs on both terminals, but the insulation between the strap from the Positive Cap and the case is VERY thin, so you have to be very careful.

I used protected cells, because if one cell dies, the protection circuit will isolate it.

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errr kindly advise to what is the needs of this question thread .... the TS wants to Parallel 48 lions together ..... not serial !!!!!!!

roadie said:

a good grounding of oneself is a basic personnel good protection when handing high voltage or amps .......

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You dont ground yourself, you never want to be grounded cus thats allows current to flow thru you and thats what you want to avoid. You ground the pack, i guess that was what you meant.


On to the OP, I would suggest Lipo since you need at least 10A and really, well made Lipos are great for huge packs. Are you going to use this to power your showerhead?

ahorton said:

I've looked at those packs.

The equivalent of what I want is http://www.batteryspace.com/polymerli-ionbattery37v50ah185wh7arate.aspx which costs $300. I bought my cells for about $70.

Theirs also has a 7A rate and I need at least 10A.

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$70 for 24 18650's. Oh boy, what brand are they?

roadie said:

errr kindly advise to what is the needs of this question thread .... the TS wants to Parallel 48 lions together ..... not serial !!!!!!!

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What on earth made you think I had 8 cells in series ????

roadie said:

i would be delighted to see u touching a circuit such as a 12v sla or this project when the circuit is alive ......

btw, "A car battery puts out 12 volts and can put hundreds of amps - have you ever heard of someone being electrocuted by one ???" =>>>>>>> no insane man will be stupid enough to handle the sla when its running ..... if u dun believe, try it yrself.....

a good grounding of oneself is a basic personnel good protection when handing high voltage or amps .......

yes, while a battery is stable enough to handle (usually) while not connect to any circuit as it will need something to "draw" volts or amps ....... But thats an different case when the circuit is alive ,,,,,,,

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I have been working on the design, building and testing of electronic systems for forty years of all sorts of voltages. I am a Licenced Electrical Contractor in Australia for installing 240 volt systems and have worked in major Electrical Substations.

For their own safety, I can only recommend that people do NOT follow roadie's advice here.

Mike .....

the forum is abt helping one another .....

since my methods might not be the best of choice (to u)..... move on ....

there's no need to flatter yrself by presenting yr resume ...... and denounce others along the way ......

keep focus on the issues of the project ......

roadie, the problem is that you seem to be making recommendations that are not only wrong, but dangerous. That's what makes people like MikeAusC respond, because this forum is about helping one another, and we don't want to see people harmed by following the wrong instructions.

I too am mystified by your advice to ground oneself. Would you care to explain your reasoning?

jasonck08 said:

$70 for 24 18650's. Oh boy, what brand are they?

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FJD, (Chinese manufacturer) They seem to work ok. I buy them by the hundred direct from the manufacturer so the price comes down but you don't get and fancy packaging.

forexer said:

Are you going to use this to power your showerhead?

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hehe, not quite. It's for a surgeon headlamp.



As for the pack:

Thanks for the comments so far. Some have been helpful.
I think I'm leaning towards charging it in a 4S6P setup. It won't be balance charged since it will balance itself when I plug it all into the the 1S24P application.

ahorton said:

I think I'm leaning towards charging it in a 4S6P setup. It won't be balance charged since it will balance itself when I plug it all into the the 1S24P application.

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Will you verify the individual 1S6P pack voltages as being relatively close to the other 3 before putting them in 1S24P? (I don't see any shortcut in making sure you catch a failed cell--which ideally is what a pack protection circuit would do vs. using individual cell PCB's). One concern I always have is when tabs are spot welded to an individually protected cell, how will you verify if the electrical arc weld and/or heat did any damage to a PCB component? When pack PCB's are used, there are solder spots for attaching leads (vs. electrical arc welding). I'm sure you have seen the cell PCB's, but think in terms of compents that may be sensitive to welding arc damage.




If you are using this to support a surgeon, personally, I would only do this with quality, name brand cells to help mitigate any individual cell problems (which is more likely with cheap, generic cells), and only charge outside and away from everyone.

This is a lot of energy, and collectively represents a bigger potential problem if any one cell goes into thermal runaway. Think of the laptop pack events to make sure you address the worst possible scenario.

roadie said:

a good grounding of oneself is a basic personnel good protection when handing high voltage or amps .......

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A good grounding of oneself when handling high voltage is a good way to die. The key to survival if there is any danger of touching a live conductor is isolation, not grounding.

So far in my experiments, the 1S6P pack voltages have been very close to each other after charging them in series with my Swallow charger.

Just to clarify about the protection circuit: There is one circuit per two 18650 cells. They came in packs of two. So the circuit is not spot welded directly to any cell.